The present invention relates to a method of controlling, by means of a cam or cams, relative position between an abrasive wheel (hereinafter simply referred to the a wheel) and a workpiece in the grinding process as well as an apparatus therefor, and, more particularly, to a method wherein the so-called gagematic system is incorporated into the so-called sizematic system so as to enjoy both high productivity of the former and high preciseness of the latter, i.e., a method wherein a minimum required spark-out grind necessary for obtaining workpieces having not only target dimension but also fine finish surface can be secured, and an apparatus therefor. This invention pertains at the same time to a method of automatically compensation for wear of a dresser (a tool for dressing a wheel), which is a prerequisite for achievement of the above purpose, and an apparatus therefor.
There have conventionally been two main systems of the grinding process for controlling finish dimension of ground workpieces; the above-mentioned so-called gagematic system and the so-called sizematic system. In the former system, workpieces are finished to a target dimension by actually measuring dimensions during the grinding operation with a final size measuring means and thereby controlling the grinding feed mechanism of the wheel and others. In the latter, workpieces are finished to a desired target dimension by controlling the wheel feed mechanism of the wheel, taking the position of the nose point of a dresser (dresser point) as a reference. In general the sizematic system is characterized in a shorter period of grinding process cycle; the gagematic system is said to be superior in high uniformity and preciseness of the finish dimension. In the cam-controlled grinding process and apparatus therefor, which is the subject matter of this invention, the sizematic system is usually employed. Controlling grinding cycles, including the final size control, by means of a cam or cams makes the control by the final size measuring difficult to be applied, which is the main reason for the employment of the sizematic system.
In the conventional sizematic system not only working wear of the wheel during the grinding but also wear of the wheel by dressing is compensated by means of the compensating means for dressing (means for displacing the chuck means which holds a workpiece as much as the amount of the external diameter reduction of the wheel by another cam system, for example) and attempts have been thereby made to maintain the finish dimension of workpieces constant. No steps have been taken, however, against change of the relative position of the dresser point to the axis of the workpiece, which change is brought about by wear of the dresser itself. Diamond, the hardest material, is used as the dresser point, nevertheless, gradual wear thereof is inevitable, which results in unavoidable variation of the final dimension of workpieces and in turn makes it impossible to continue grinding processes of uniform and high precision.
The cam-controlled grinding machines are mostly used for internal grinding of smaller diameter bores, so the wheel diameter is small. It means the wear rate of the wheel in the grinding is rather great; it is normal, therefore, to carry out dressing of the wheel once per one workpiece (in some cases several times of dressing are done per one workpiece, in some other cases only one dressing is sufficient for several pieces of works). The more the number of frequency of dressing, the larger becomes the wear of the dresser point.
During the "spark-out" (a phenomenon wherein sparks by grinding cease to come out) time in a grinding cycle on a grinding machine, the abrasive wheel will be still subject to grinding feed, though only slightly, up until deflection of the wheel spindle (as well as workpiece deflection) has disappeared, irrespective of further increase of the grinding feed movement due to the cam profile; thus final dimension of the workpiece is still subjected to changing. In particular, internal grinding machines for rather small internal diameter bores normally have a wheel spindle of smaller diameter than the diameter of the wheel which is originally small, so the deflection of the wheel spindle due to grinding force will naturally be great, so that restoration of the deflection during the sparkout step is an important factor requiring attention. Considering the merit of remarkably improving the finish surface roughness during the continued grinding period of sparkout, i.e., a certain period of time before sparks have completely been ceased (hereinafter simply referred to as sparkout time) by restoration of deflection of the wheel spindle, the problem is critical that wear of the dresser point (including another problem that increasing of the area of the pointed end of the wheel affects the wheel surface and in turn the surface character of the workpiece) continuously increases dimensions of the workpieces and thereby makes the securing of time, during which fine finish surface by the sparkout grinding is expected, extremely difficult. In the sizematic system compensation for the wheel dressing has been conventionally practiced and uniformity of finish dimension of workpieces has been thereby attained in a way. The continuous tendency of increasing the finish dimension of workpieces due to wear of the dresser point mentioned above has by no means been compensated. The sizematic system is incapable of compensation for variation in the deflection of the wheel spindle in response to changing of the shape of the dresser point, non-uniformity of the wheel, variation in workpiece material, unstable deflection set forth hereinafter, etc., and in turn variation in time-of-deflection-restoration of the wheel spindle in the sparkout time.
Further attention is required to the fact that repeated dressing tends to decrease the wheel diameter, resulting in decrease of the peripheral velocity and increase of resistance for grinding even for a constant rate of feeding which resistance will increase deflection of the spindle and make the deflection inconstant or unstable. In the conventional method, operators have had to check finish dimensions of workpieces, finish surface roughness and manually to adjust on occasion the position of the dresser or others, which largely depends on hunch or experience of the operator and is far from stable production of workpieces of uniform dimension. Diversification of causes thereof has been preventing the solution of the problem.